Hi, Next part as promised. A working item for me is a deadlock detector for MVCC transactions [1]. The message is structured in 2 parts. First is an analysis of the current state of affairs and possible options to go. Second is a proposed option. First part is going to be not so short so some might prefer to skip it.
ANALYSIS The immediate question is "why we cannot use an existing deadlock detector?". The differences between classic and MVCC transactions implementation is the answer. Currently a collection of IgniteTxEntry is used for detection. But such collection is not maintained for MVCC transactions. So, it will not work out of box. Also it looks like that current distributed iterative approach cannot be low latency it the worst case because of doing possibly many network requests sequentially. So, what options do we have? Generally we should choose between centralized and distributed approaches. By centralized approach I mean existence of a dedicated deadlock detector located on a single node. In the centralized approach we can face difficulties related to failover as a node running deadlock detector can fail. In the distributed approach extra network messaging overhead can strike because different nodes participating in a deadlock can start detection independently and send redundant messages. I see some aspects which make sense for choosing implementation. Here they are with an approach that is better (roughly speaking) in parentheses: * Detection latency (centralized). * Messaging overhead (centralized). * Failover (distributed). And also having a park of deadlock detectors sounds not very good. I hope that it is possible to develop a common solution suitable for both kinds of transactions. I suggest to pilot new solution with MVCC and then adopt it for classic transactions. PROPOSAL Actually I propose to start with an centralized algorithm described by Vladimir in the beginning of the thread. I will try to outline main points of it. 1. Single deadlock detector exists in the cluster which maintains transaction wait-for graph (WFG). 2. Each cluster node sends and invalidates wait-for edges to the detector. 3. The detector periodically searches cycles in WFG and chooses and aborts a victim transaction if cycle is found. Currently I have one fundamental question. Is there a possibility of false detected deadlocks because of concurrent WFG updates? Of course there are many points of improvements and optimizations. But I would like to start from discussing key points. Please share your thoughts! [1] https://issues.apache.org/jira/browse/IGNITE-9322 ср, 14 нояб. 2018 г. в 15:47, ipavlukhin <vololo...@gmail.com>: > > Hi Igniters, > > I would like to resume the discussion about a deadlock detector. I start > with a motivation for a further work on a subject. As I see current > implementation (entry point IgniteTxManager.detectDeadlock) starts a > detection only after a transaction was timed out. In my mind it is not > very good from a product usability standpoint. As you know, in a > situation of deadlock some keys become non-usable for an infinite amount > of time. Currently the only way to work around it is configuring a > timeout, but it could be rather tricky in practice to choose a > proper/universal value for it. So, I see the main point as: > > Ability to break deadlocks without a need to configure timeouts explicitly. > > I will return soon with some thoughts about implementation. Meanwhile, > does anybody have in mind any other usability points which I am missing? > Or is there any alternative approaches? > > On 2017/11/21 08:32:02, Dmitriy Setrakyan <d...@apache.org> wrote: > > On Mon, Nov 20, 2017 at 10:15 PM, Vladimir Ozerov <vo...@gridgain.com>> > > wrote:> > > > > > It doesn’t need all txes. Instead, other nodes will send info about> > > > suspicious txes to it from time to time.> > > >> > > > > I see your point, I think it might work.> > > -- Best regards, Ivan Pavlukhin
